Normally, seawater is used for ballast water and, as a result, a large amount of bacteria and microorganisms living in the sea are present in the seawater used as the ballast water. When this ballast water is discharged into the seawater at a different location such as a ship's destination, a large amount of normative microorganisms are discharged in the seawater at that destination, which adversely impacts the ecological system in that area.
One method to prevent such adverse impact on the ecological system is—while a ship is at sea during its voyage—to exchange the ballast water with seawater that contains relatively few bacteria and microorganisms. But this method, in which the ballast tank is emptied and then seawater is pumped thereinto, also can cause problems, including danger due to the instability of the ship and difficulty of doing that pumping in bad weather.
Another method, in which new seawater is pumped into the ballast tank so that old seawater is drained out of the ballast tank, is difficult to implement because the amount of seawater required for exchanging 95% or more of the ballast water is about three times as large as a capacity of the ballast tank. For instance, if a ballast tank has a capacity of 60,000 tons, it is necessary to pump 180,000 tons of seawater into the tank to replace the old seawater with new seawater.
In February 2004, the International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments. The purpose of this international convention is to ensure the protection of oceanic ecosystems and smooth international marine transport and, through the control and management of ships' ballast water and sediments, to protect the environment, human health, property, and resources from damage caused by the transfer of harmful aquatic organisms and pathogens.
The ballast-water performance standards are as follows:                Zooplankton—less than 10 viable organisms per cubic meter        Phytoplankton—less than 10 viable organisms per milliliter (ml)        Toxicogenic Vibrio cholerae—less than 1 colony-forming unit (cfu) per 100 ml        Escherichia coli—less than 250 cfu per 100 ml        Intestinal Enterococci—less than 100 cfu per 100 ml        
In addition to the above-described methods for replacing ballast water while at sea, there is a method for sterilizing ballast water through arc-electric discharge by applying a high-voltage pulse when the ballast water is sucked into or discharged from a ballast tank (see Patent Document 1).
Other methods include using the steam heat of a boiler, using the DNA destructive action of ultraviolet rays, and using ozone to effect oxidative decomposition of cell membranes (see Patent Document 2).
The applicant of the invention of the current application has previously proposed a fluid-mixing apparatus that efficiently mixes two fluids such as a gas and a liquid by causing a swirling flow in a fluid passing through a pipe so as to cause cavitation (see Patent Document 3), and an ozone-reaction apparatus that uses ozone for purification (see Patent Document 4). This fluid-mixing apparatus, which is also referred to as an in-line mixer, can sterilize a large amount of ballast water in a short time, and so it has been used in various experiments.
According to these experiments, the in-line mixer is effective at sterilizing ballast water because it can destroy the cell membranes of planktons by the combined effect of three actions, i.e., (1) pressurization by the swirling flow that presses the ballast water against the inside surface of a pipe, (2) collisions achieved by causing the swirling flow of the ballast water to crash against a multiplicity of projections that are formed on the inside surface of a pipe, and (3) shock waves due to cavitation. Such shock waves are generated as follows. A cavity generated at a center of the swirling flow of the ballast water is under low pressure; therefore, bubbles form, and when the bubbles grow to sufficient size they burst due to the higher pressure of the ballast water surrounding the cavity. This dynamical action is effective for sterilizing planktons and larva of tens to hundreds of microns in size. To better assure the desired effect, an in-line mixer that can cause a faster swirling flow is sought.
These experiments have shown that the dynamical action of the in-line mixer is not capable of sterilizing cholera bacillus and E. coli, because the sizes of the cholera bacillus and E. coli, which are 1 micron to 5 microns in size, are too fine to sterilize. Therefore, sterilization must rely on contact between a bactericide and those bacteria, and ozone-sterilization having a sterilization action seven times as great as that of chlorine is desired. To this end, it is necessary to instantaneously achieve mixing of a gas (bactericide) and liquid (ballast water), or micromixing of two liquids (bactericide and ballast water).
Although the in-line mixer is excellent in such mixing ability, further improvement of mixing performance is sought in order to surely sterilize microorganisms in the ballast water.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-192161    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-160437    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2000-354749    Patent Document 2: Japanese Unexamined Patent Application Publication No. Heisei 7 (1995)-124577